There's no particular reason opener won't have 2 cds, and there's just not going to be a huge number of times that 3c is going to make when 1nt fails, compared to the reverse. Particularly when it is bidding on 5. Book bid should be to pass, allow to simulate and bid 3c if 4-6+.
Just saw it go down for the umpteenth time on this sequence when 1nt is cold.